Match math function behavior to DX9 implementation

[dpw13]

See https://github.com/projectM-visualizer/projectm/issues/724

[kblaschke]

        else if (String_Equal(functionName, "pow"))
        {
            HLSLExpression* argument[2];
            if (GetFunctionArguments(functionCall, argument, 2) != 2)
            {
                Error("%s expects 2 arguments", functionName);
                return;
            }
            // See rsqrt above regarding abs().
            m_writer.Write("pow(abs(");
            OutputExpression(argument[0]);
            m_writer.Write("),");
            OutputExpression(argument[1], &functionCall->function->argument->type);
            m_writer.Write(")");
            handled = true;
        }

Adding this after handling log10 in GLSLGenerator.cpp fixes "stormy sea", as pow() is also affected by the NaN madness.